Concentration of minerals



6 c. A. HOLLINGSWORTH 3,293,519

CONCENTRATION OF MINERALS Filed Oct. 25, 1963 .l s v A l4 An Pervious I I Partition Water ?ll 20 Non-flout.

- Tails Nob-flout. INVENTOR Underflow Cllnron A. Hollmgsworfh ATTORNEYS United States Patent 3,298,519 CONCENTRATION OF MINERALS Clinton A. Hollingsworth, Lakeland, Fla, assignor, by

mesne assignments, to The Borden Company, a corporation of New Jersey Filed Oct. 23, 1963, Ser. No. 318,393 8 Claims. (Cl. 209-165) This invention relates to the concentration of minerals and other particulate material by froth flotation, and more particularly it relates to the separation by fractionating flotation of a mixture of relatively floatable and relatively non-floatable mineral particles.

In conventional processes for the separation and concentration of minerals and other particulate material by froth flotation, an aqueous suspension or pulp of a mixture of floatable and non-floatable materials, and, usually, flotation reagents for the floatable material, is subjected to vigorous agitation and aeration in a suitable flotation machine so that the entire body of the pulp is a substantially uniform mixture of suspended solid particles, air bubbles and flotation reagents with a froth or bubble column floating on top of the agitated pulp. The feed mixture of particulate material is normally introduced into one end of the flotation machine, and the agitated pulp travels or progresses in an essentially horizontal direction to the pulp discharge at the opposite end of the machine. The agitated pulp, of course, becomes increasingly depleted in floatable mineral values as the pulp progresses from the feed end to the discharge end of the machine. However, the distribution of the floatable and non-floatable mineral particles in the aqueous pulp at any point along the length of the machine is substantially uniform throughout the entire vertical section of the agitated pulp at that point. As a result it is obvious that the separation of floatable and non-floatable particles and the concentration of the floatable particles does not take place to any appreciable extent within the agitated aqueous pulp itself but rather it takes place at the interface of the agitated pulp and the overlying froth and within the froth or bubble column itself' As more 7 and more of the floatable particles are concentrated in the froth or bubble column, the concentration of these particles throughout the pulp itself is correspondingly reduced. When the maximum practical degree of separation has been obtained, the froth rich in floatable mineral values and the pulp depleted in floatable mineral values cellent separation in the case of ores or other particulate material the relative floata-bility of the particles of which is quite marked. However, in the case of ores or other materials comprising mixtures of particles less amenable to separation by froth flotation, the efficiency of separation and degree of recovery of the desired concentrate leaves something to be desired.

1 have now discovered a new process for separating mixtures of relatively floatable and relatively non-floatable particles in which the desired separation takes place almost exclusively in the aqueous pulp (or, more accurately, in a column of liquid corresponding roughly to the aqueous pulp of the conventional prior art processes). Flotation conditions in my new process are so controlled that the separation of the particles is effected by a fractionating flotation mechanism which was heretofore unlmown in the flotation art and which is characterized by a progressive decrease in the concentration of the floatable particles from a maximum near the top of the body of aqueous pulp to a minimum adjacent the bottom of the pulp body and by a corresponding progressive decrease in the concentration of the non-floatable particles from p a maximum near the bottom of the pulp body to a minimum adjacent the top of the pulp body. My fractionating flotation process results in a significant improvement in the efliciency of separation and degree of recovery of the desired concentrate, particularly in the case of ores that are less than ordinarily amenable to beneficiation by froth flotation, a single flotation machine adapted to carry out my new process consistently outperforming a battery of four conventional sub-aeration impeller-type machines in a series of comparative tests involving the flotation of a phosphate ore.

My new fractionating flotation process for separating a mixture of relatively floatable and relatively non-floatable particulate material in the presence of a flotation reagent for the relatively floatable material comprises establishing a vertical column of liquid of substantial height, and specifically at least about six feet in height, introducing aerating air into the column of liquid, and introducing the mixture of particles into the aerated column of liquid at least about two feet below the surface of the liquid column, and preferably at a depth equal to at least one third the height of the column. A substantial portion of the aerating air is introduced into the column below the point at which the mixture of particles is introduced thereinto. A heavy fraction predominantly comprising relatively non-floatable particles of the feed material is withdrawn from the lower portion of the liquid column, and a light fraction predominantly comprising relatively floatable particles of the feed material is withdrawn from the top of the aerated column. The rate of introduction of feed material, liquid and aerating air into the liquid column and the rate of removal of liquid and said heavy and light fractions from the column are controlled so that the net rate of upward flow of liquid in the column is less than the average rate at which the relatively nonfloa-table particles sink in a stationary body of the liquid and the net rate of downward flow of liquid in the column is less than the average rate at which the relatively floatable mineral particles rise toward the surface of an aerated stationary body of the liquid. As a result, the concentration of the relatively floatable particles in the liquid column progressively decreases from a maximum at the 'top of the column to a minimum adjacent the bottom of the column and the concentration of the relatively nonfloatable mineral particles progresively decreases from a maximum adjacent the 'bottom of the column to a minimum adjacent the top of the column, the separation of the feed material into a light or float fraction and a heavy or sink fraction taking place in a manner analogous to the progressive separation of lights and heavies in a fractionatin g distillation column.

The fractionating flotation process may be carried out in any type of flotation apparatus in which the requisite flotation conditions can be established. Thus, the apparatus must be provided with a tank or flotation compartment of sufficient depth to permit the establishment therein of a vertical column of liquid of substantial height, and as the column of liquid is at least 6 feet and preferably is from 10 to 30 feet in height the flotation compartment should be of corresponding size. The apparatus should have means for introducing the mixture of relatively float able and non-floatable particulate matter into the vertical column of liquid a susbtantial distance below the surface of the column, that is at a point located at least 2 feet below the surface of the column and preferably at a point located between one-third and two-thirds of the depth of the column. In addition, means for aerating the liqu..d column must be provided, and a substantial portion (at least and preferably about 20%) of the aerating air must be introduced into the column below the point at which the feed mixture is introduced thereinto. The aerating air may be introduced, for example, by means of perforated rubber tubing or by means of a mechanical aerator of essentially conventional design. Means must be provided for withdrawing the light or float fraction from the top of the liquid column and for withdrawing the heavy or sink fraction from the bottom portion of the vertical column of liquid, and means for controlling the rate of introduction of feed, aerating air and liquid into the column and the rate of removal of said light and heavy fractions from the column must be provided so that the vertical velocity of liquid within the column either upwardly or downwardly is within the limits prescribed. Apparatus suitable for carrying out the process of my invention are shown schematically in the accompanying drawing of which FIG. 1 is a schematic sectional view of a hydraulic pneumatic flotation cell adapted to carry out my fractionating flotation process,

FIG. 2 is a schematic sectional view of a flotation cell provided with mechanical aeration impellers, and

FIG. 3 is a schematic representation of the light and heavy particle distribution within a flotation cell adapted to carry out my new process.

The hydraulic-pneumatic flotation apparatus shown schematically in FIG. 1 of the drawing comprises a deep, essentially unobstructed flotation compartment 10 in which a vertical column of liquid 111'. at least six feet, and preferably between ten and thirty feet, in depth is established. The apparatus is provided with liquid inlet means 12 for introducing make-up or fluidizing liquid into the flotation compartment advantageously near the bottom thereof and with valve means 13 for controlling the amount of liquid introduced into the flotation compartment. Aerating means 14 for introducing aerating air into the column of liquid are provided preferably at several different levels between the bottom and the top of the flotation compartment, and valve means 15 are provided for controlling the amount of aerating air introduced into the column of liquid at each level. The aerating means 14 advantageously comprises a plurality of perforated rubber tubes of the type shown in FIG. 4 of US. Patent 2,758,714.

Feeding means, for example the feed pipe 16, is provided for introducing the mixture of relatively floatable and relatively non-floatable particulate material into the flotation compartment a substantial distance below the surface of the column of liquid 11. Specifically, I have found that to obtain fractional flotation in accordance with my invention, the feed material should be introduced at least two feet, and preferably between one-third and twothirds of the total depth of the liquid column, below the surface of the liquid, and the feeding means is adapted to accomplish this result. The feed material is usually treated or conditioned with one or more flotation reagents, and advantageously it is mixed with water (or other flotation medium) to form a pulp or slurry, prior to being in troduced into the flotation apparatus. In addition, liquid can be added to the top of the apparatus to help control the dilution and direction of flow of the flotation medium. The apparatus is provided at its upper end with an overflow weir or lip 18 at which point the light fraction comprising essentially the relatively floatable particulate material is removed from the top of the column of liquid, and it is further provided with an outlet pipe 19 having a control valve 20 through which the heavy fraction comprising essentially the relatively non-floatable particulate material is withdrawn from the flotation tank.

The mechanically aerated flotation apparatus shown schematically in FIG. 2 comprises a flotation compartment 10 adapted to contain a vertical column of liquid medium 11 of substantial depth, water inlet means 22 for introducing make-up or fluidizing liquid for the flotation medium, feeding means 16 for introducing the feed mixture into the flotation tank at a point a substantial distance below the surface of the liquid column, light or float overflow means 18 adapted to remove the fraction composed predominantly of relatively floatable material, and heavy or sink outlet means 24 for withdrawing the fraction composed predominantly of relatively non-floatable material. In the apparatus shown in FIG. 2 the make-up or fluidizing liquid for the flotation medium is introduced at the bottom of the flotation compartment and enters the vertical column of liquid through a pervious partition 26, the heavy fraction (referred to as tails in the drawing) being removed from the apparatus at a point just above the pervious partition. The apparatus is further provided with a mechanical aerator advantageously of the impeller type. The aerator comprises a plurality of aerating impellers 28 mounted on a hollow rotatable shaft 29 that is rotated by a motor 30, aerating air being introduced into the liquid through the holes or openings in the hollow drive shaft 283 at each impeller which in turn disperses the air and thus aerate-s the liquid. The impellers 28 are preferably of a type that does not create a strong pumping action, and air under pressure can be introduced into the shaft 29 to supplement that normally drawn in by the impellers 28. The impellers, of course, agitate the flotation medium in the vicinity of each impeller, but this local agitation of the flotation medium does not appreciably affect the over-all vertical movement of liquid within the flotation compartment and therefore such local agitation does not interfere with the fractiona-ting flotation process of my invention.

The means provided for controlling the rate of introduction of aerating air, make-up liquid and feed material into the flotation compartment and for withdrawing the light and heavy fractions therefrom make it possible to control the vertical velocity of the liquid within the flotation compartment so that the net rate of upward flow of liquid in the tank is less than the average rate at which the relatively non-floatable particles sink in a stationary body of the liquid flotation medium and so that the net rate of downward flow of liquid in the column is less than the average rate at which the relatively floatable particles and air bubbles rise toward the surface of an aerated stationary body of the liquid. As a result of this control of the vertical velocity of the liquid flotation medium, as

well as the substantial height of the column of flotation medium and the point of introduction of the feed material thereint-o, the particulate feed material is separated into a light fraction composed predominantly of relatively floatable mineral particles and a heavy fraction composed predominantly of non-floatable mineral particles in a manner analogous to the fractional separation of lights and heavies that occurs in a fractional distillation column. The distribution of floatable and non-floatable particles as observed through the transparent plastic side wall of a full size flotation cell in which my process was being carried out is illustrated schematically in FIG. 3 of the drawing, the solid dots representing the relatively non-floatable particulate material and the circles (0) representing the relatively floatable particulate material.

The terms relatively floatable and relatively nonfloatable particulate material refer to the particles of a feed mixture that can be separated by flotation techniques into a float fraction and a sink fraction such, for example, as mixtures of metallic sulfides and oxidic gangue, of apatitic phosphates and silica, and the like. Moreover, it will be understood that while the float fraction, which ,is composed predominantly of relatively floatable particulate material, is referred to for convenience as the light fraction and the sink fraction, which is composed predominantly of relatively non-floatable particulate material, is referred to as the heavy fraction, the particles comprising both fractions are actually of greater density than the flotation medium, and it merely mixed with the flotation medium without aeration the particles of both fractions would settle to the bottom of the flotation medium. However, when the flotation medium is aerated, and especially when the feed mixture is treated or conditioned with a flotation reagent for the floatable particles, the relatively floatable particles tend to adhere or attach themselves to air bubbles rising in the aerated column of liquid, and thus these particles are gradually transported to the upper surface of the liquid column where the air bubbles with the floatable particles adhering thereto are removed as described. Moreover, by changing the type of flotation reagent from, say, an anionic to a cationic reagent, the particles which comprise the non-float or heavy fraction when one type reagent is employed may comprise the float or light fraction when the other type of reagent is employed, both phosphate particles and silica particles being fioatable or non-floataole in this manner in my process. The separation of the feed mixture of relatively floatable and relatively non-floatable particles into socalled light and heavy fractions is therefore the result of a true flotation mechanism and is not to be confused with the gravity separation that can be effected between particles of appreciably different densities or s-pecific gravities by means of a liquid medium of an intermediate specific gravity.

The following examples are illustrative but not limitative of the practice of my invention:

EXAMPLE I A Tenoroc flotation feed comprising essentially a conditioned mixture of apatite phosphate and silica particles was fed to a bank of four standard mineral separation (MS) sub-aeration flotation machines connected in series, and a phosphate concentrate was floated and recovered in the usual manner. The same conditioned feed material was fed to a fr-actionating flotation (FF) cell adapted to carry out my invention, the feed material being introduced into the cell at a point 6 feet below the surface of liquid in the cell and the cell being provided with 14 feet of aeration (i.e. aerating air being introduced at a point 14 feet below the surface of the liquid in the cell and at several different points therebetween). The feed material contained 29.94% by weight BPL (bone phosphate of lime or essentially tricalcium phosphate). The phosphate concentrate produced by the four conventional MS machines contained 69.92% BPL and 10.63% by weight of insoluble material (mainly silica), and the fractionating flotation cell of my invention produced a phosphate concentrate containing 71.85% BPL and 7.62% insols. These results are summarized in the following table:

EXAMPLE II A conditioned Tenoroc flotation feed of apatite phosphate and silica particles was fed to a bank of four standard mineral separation flotation machines connected in series, and an MS phosphate concentrate was floated and recovered. The same conditioned feed material was also introduced into a fractionating flotation cell at a point a substantial distance below the surface of the liquid therein in accordance with my invention, and a FF phosphate concentrate was floated and recovered. The feed material contained 35.19% by weight BPL. The phosphate con- 6 centrate produced by the four conventional MS machines contained 73.42% BPL and 8.40% by Weight of insoluble material, and the phosphate concentrate produced by the fractionating flotation cell of my invention contained 77.01% BPL and 4.30% insols. These results are summarized in the following table:

EXAMPLE III A conditioned mixture of apatite phosphate and silica particles was separated on a conventional Tenoroc spray belt into a rougher phosphate concentrate and silica-containing tailings, and the rougher belt phosphate concentrate was then fed to a conventional cleaner belt to obtain a cleaner belt phosphate concentrate. The same rubber belt phosphate concentrate was also fed to the center of a fractionating flotation cell in accordance with my invention to obtain a PE cleaner phosphate concentrate. The phosphate concentrate produced by the cleaner belt contained 64.79% BPL and 18.22% by weight of insoluble material, and the phosphate concentrate produced by the fractionating flotation cell of my invention contained 76.53% BPL and 3.35% insols. These results are summarized in the following table:

A Tenoroc reagentized belt feed containing 49.76% BPL was beneficiated by a conventional Tenoroc spray belt to obtain a rougher belt phosphate concentrate containing 58.29% BPL, and the rougher belt concentrate was further beneficiated on a second Tenoroc spray belt to obtain a cleaner belt phosphate concentrate containing 72.82% BPL and 6.78% insoluble matter. The same reagentized belt feed was also introduced into a fractionating flotation cell provided with 14 feet of aeration in accordance with my invention to obtain an FF phosphate concentrate containing 75.17% BPL and 4.37% insols. These results are summarized in the following table:

EXAMPLE V A Tenoroc reagentized belt feed containing 49.01% BPL was beneficiated on a conventional Tenoroc spray belt to obtain a rougher belt phosphate concentrate containing 58.30% BPL and 26.28% insoluble matter. The

TABLE Concentrate Feed, percent L Percent Percent BPL lnsols Cleaner Belt 58. 3O 73. 84 6. 30 Cleaner FF Cell -s 58. 30 76. 68 3. 31

The test results reported in the foregoing specific examples demonstrate that the fractionating flotation column operated in accordance with my invention consistently produced a higher grade phosphate concentrate than did a bank of four conventional mineral separation flotation machines or a conventional spray belt circuit.

I claim:

1. Fractionating flotation process for separating a mixture of relatively floatable and relatively non-floatable particulate material which comprises: establishing a vertical column of liquid at least about six feet in height; aerating the column of liquid; introducing the mixture of particles into said aerated column of liquid at least about two feet below the surface of the liquid column, there being no significant decrease in the horizontal cross-sectional area of the column of liquid from above the point at which the mixture of particulate material is introduced into the column of liquid to below the surface of the liquid column; introducing a substantial portion of the aerating air into the column below the point at which the mixture of particulate material is introduced thereinto; withdrawing a heavy fraction predominantly comprising relatively non-floatable particles of the feed material from the lower portion of the liquid column; removing a light fraction predominantly comprising relatively floatable particles of the feed material from the top of the aerated column; and controlling the rate of introduction of feed material and liquid into the liquid column and the removal of liquid and said heavy and light fractions from said column so that the net rate of upward flow of liquid in the column is less than the average rate at which the relatively non-floatable particles sink in a stationary body of said liquid and the net rate of downward flow of liquid in the column is less than the average rate at which the relatively floatable particles and air bubbles rise toward the surface of an aerated stationary body of said liquid, whereby the concentration of the relatively floatable particles progressively decreases from a maximum at the top of the liquid column to a minimum adjacent the bottom of the column and the concentration of the relatively nonfloatable particles progressively decreases from a maximum adjacent the bottom of the column to a minimum at the top of the column to effect separation of the mixture of particulate material by fractionating flotation.

2. Fractionating flotation process for separating a mixture of relatively floatable and relatively non-floatable particulate material in the presence of a flotation reagent for said relatively floatable material which comprises: establishing a vertical column of liquid at least about six feet in height; aerating the column of liquid at several points below the surface of the liquid column; introducing the mixture of particles into said aerated column of liquid at least two feet below the surface of the liquid column, there being no significant decrease in the horizontal crosssectional area of the column of liquid from above the point at which the mixture of particulate material is intro-duced into the column of liquid to below the surface of the liquid column; introducing at least 10% of the aerating air into the column below the point at which the mixture of particulate material is introduced thereinto; introducing fluidizing liquid into said column of liquid at at least one point; withdrawing a heavy fraction predominantly comprising relatively non-floatable particles of the feed material from the lower portion of the liquid column; removing a light fraction predominantly comprising relatively floatable particles of the feed material from the top of the aerated column; controlling the rate of introduction of feed material and fluidizing liquid into the liquid column and the removal of liquid and said heavy and light'fractions from said column so that the net rate of upward flow of liquid in the column is less than the average rate at which the relatively non-floatable particles sink in a stationary body of said liquid and the net rate of downward flow of liquid in the column is less than the average rate at which the relatively floatable particles and air bubbles rise toward the surface of an aerated stationary body of said liquid, whereby the concentration of the relatively floatable particles progressively decreases from a maximum at the top of the liquid column to a minimum adjacent the bottom of the column and the concentration of the relatively non-floatable particles progressively decreases from a maximum adjacent the bottom of the column to a minimum at the top of the column.

3. Fractionating flotation process for separating a mixture of relatively floatable and relatively non-floatable particulate material in the presence of a flotation reagent for said relatively floatable material which comprises: establishing a vertical column of liquid between about 6 and 30 feet in height; aerating the column of liquid at several points below the surface of the liquid column; introducing the mixture of particles into said aerated column of liquid at a point located below the surface of the liquid column a distance equal to about one third to tWo thirds the height of the liquid column, there being no significant decrease in the horizontal cross-sectional area of the column of liquid from above the point at which the mixture of particulate material is introduced into the column of liquid to below the surface of the liquid column; introducing between 10% and 50% of the aerating air into the column below the point at which the mixture of mineral particles is introduced thereinto; introducing fluidizing liquid into said column of liquid at at least one point below the surface of the liquid column; withdrawing a heavy fraction predominantly comprising relatively nofloatable particles of the feed material from the lower portion of the liquid column; removing a light fraction predominantly comprising relatively floatable particles of the feed material from the top of the aerated column; controlling the rate of introduction of feed material and fluidizing liquid into the liquid column and the removal of liquid and said heavy and light fractions from said column so that the net rate of upward flow of liquid in the column is less than the average rate at which the relatively non-floatable particles sink in a stationary body of said liquid and the net rate of downward flow of liquid in the column is less than the average rate at which the relatively floatable particles and air bubbles rise toward the surface of an aerated stationary body of said liquid, whereby the concentration of the relatively floatable particles progressively decreases from a maximum at the top of the liquid column to a minimum adjacent the bottom of the column and the concentration of the relatively nonfloatable mineral particles progressively decreases from a maximum adjacent the bottom of the column to a minimum adjacent the top of the column to effect separation of the mixture of particulate material by fractionating flotation.

4. The fractionating flotation process according to claim 3 in which the liquid column is between about 10 and 30 feet in height.

5. The fractionating flotation process according to claim 3 in which about 20% of the aerating air is introduced into the liquid c-olumn below the point at which the mixture of particulate material is introduced thereinto.

6. Apparatus for separation by fractionating flotation of a mixture of relatively floatable and relatively non-floatable particulate material Which comprises: a flotation compartment adapted to contain a vertical column of liquid at least about six feet in height; means for introducing the mixture of particles into the flotation compartment at least about two feet below the surface of a liquid column contained in said compartment, there being no significant decrease in the horizontal cross-sectional area of the flotation compartment from above the point at which the mixture of particles is introduced into said compartment to below the surface of a liquid column contained in said compartment; means for aerating a liquid column contained in the flotation compartment and for introducing a substantial portion of said aerating air into said compartment below the point at which the mixture of particles is introduced thereinto; means for introducing fluidizing liquid into the flotation compartment; means for withdrawing a heavy fraction predominantly comprising relatively non-floata-ble particles of the feed material from the lower portion of the flotation compartment; means for removing a light fraction predominantly comprising relatively floatable particles of the feed material from the top of the flotation compartment; and means for controlling the rate of introduction of feed material and liquid into the flotation compartment and the rate of removal of liquid and said heavy and light fractions from said compartment so that the net rate of upward flow of a liquid contained in said compartment can be maintained at less than the average rate at which the relatively non-floatable particles would sink in a stationary body of said liquid and so that the net rate of downward flow of a liquid contained in said compartment can be maintained at less than the average rate at which the relatively floatable mineral particles and air bubbles would rise toward the surface of an aerated stationary body of said liquid.

7. Fractionating flotation apparatus according to claim 6 in which the flotation compartment is at least 6 feet in height and in which the means for introducing the mixture of particulate material into the flotation compartment is located at least 2 feet below the top of the compartment.

8. Fractionating flotation apparatus according to claim 6 in which the flotation compartment is between about 6 to 30 feet in height and in which the means for introducing the mixture of particulate material into the flotation compartment is located below the top of the compartment a distance equal to between about one third to two thirds the height of the compartment.

Taggert, Arthur F.: Elements of Ore Dressing, New York, N.Y., John Wiley & Sons, Inc, 1951, p. 308 relied HARRY B. THORNTON, Primary Examiner. L. EATHERTON, Assistant Examiner. 

1. FRACTIONATING FLOTATION PROCESS FOR SEPARATING A MIXTURE OF RELATIVELY FLOATABLE AND RELATIVELY NON-FLOATABLE PARTICULATE MATERIAL WHICH COMPRISES: ESTABLISHING A VERTICAL COLUMN OF LIQUID AT LEAST ABOUT SIX FEET IN HEIGHT; AERATING THE COLUMN OF LIQUID; INTRODUCING THE MIXTURE OF PARTICLES INTO SAID AERATED COLUMN OF LIQUID AT LEAST ABOUT TWO FEET BELOW THE SURFACE OF THE LIQUID COLUMN, THERE BEING NO SIGNIFICANT DECREASE IN THE HORIZONTAL CROSS-SECTIONAL AREA OF THE COLUMN OF LIQUID FROM ABOVE THE POINT AT WHICH THE MIXTURE OF PATICULATE MATERIAL IS INTRODUCED INTO THE COLUMN OF LIQUID TO BELOW THE SURFACE OF THE LIQUID COLUMN; INTRODUCING A SUBSTANTIAL PORTION OF THE AERATING AIR INTO THE COLUMN BELOW THE POINT AT WHICH THE MIXTURE OF PARTICULATE MATERIAL IS INTRODUCED THEREINTO; WITHDRAWING A HEAVY FRACTION PREDOMINANTLY COMPRISING RELATIVELY NON-FLOATABLE PARTICLES OF THE FEED MATERIAL FROM THE LOWER PORTION OF THE LIQUID COLUMN; REMOVING A LIGHT FRACTION PREDOMINANTLY COMPRISING RELATIVELY FLOATABLE PARTICLLES OF THE FEED MATERIAL FROM THE TOP OF THE AERATED COLUMN; AND CONTROLLING THE RATE OF INTRODUCTION OF FEED MATERIAL AND LIQUID INTO THE LIQUID COLUMN AND THE REMOVAL OF LIQUID AND SAID HEAVY AND LIGHT FRACTIONS FROM SAID COLUMN SO THAT THE NET RATE OF UPWARD FLOW OF LIQUID IN 